Hearing assistive systems are used to assist the hearing impaired to best compensate for a user's hearing loss profile. Hearing assistive systems can include hearing aids, cochlear implants, and related devices. In simple terms a hearing aid is a device worn by a user to pick up and amplify sound and provide it to a user's ear canal. Hearing aid technology has continually improved, resulting in ever smaller devices which are often categorized by how they fit about a user's ear. For example, some common hearing aid styles listed by decreasing size include behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC), mostly-in-the-canal (MIC) and completely-in-the-canal (CIC) hearing aids.
Hearing aids worn outside the ear and cochlear implant input devices operate in either a microphone mode, in which sound waves incident upon the device are converted to electrical energy, or a telecoil mode, in which magnetic energy is converted to electrical energy. In either mode, the resultant electrical signal is subsequently amplified, processed, and output to the user. When a hearing aid operates in an environment that includes a modulated RF field or a fluctuating magnetic field, undesired interference may be induced in the hearing aid circuitry as the varying fields are detected and processed as electrical signals by the hearing aid.
Mobile communication devices have now become commonplace, and interference generated by these devices continues to pose challenges for hearing aid designers. Use of a mobile communication device commonly involves placing it close to an ear so that sounds from a speaker can be heard. This placement often leads to a hearing aid of a user being located within a strong, interference-inducing RF field of the communication device. The problem is further exacerbated by the fact that interference generated by the now common digital communication devices such as digital cell phones is generally more obtrusive than that caused by their analog counterparts.
Digital wireless telephones transmit over a wireless network via radio waves. The radio waves generated by the digital telephone are typically detected and demodulated by the hearing aid circuitry, thereby introducing an interference signal to the hearing assistive device. The interference signal is then amplified, processed, and delivered to the user along with the desired signal. As a result, the audible quality of the desired signal is diminished. Digital wireless devices that employ time division multiplexed modulation schemes often generate interference due to the on/off keying of their modulation envelopes. The pulsing of the transmissions may produce interference at the fundamental frequencies associated with the pulse rates, as well as at the associated harmonic frequencies across the audible spectrum. Interference may also be produced by RF energy picked up by components of hearing assistive devices, such as a telecoil in a hearing aid.
The digital telephone's electronics, such as the backlighting, the display, the keypad, the battery leads and the circuit board often also generate pulsed magnetic fields. The resultant magnetic field energy typically impacts for example, a hearing aid's wiring and interconnections, to generate interference at the hearing aid. This type of interference, often referred to as baseband magnetic interference, is also converted to an electrical signal that is then processed by the hearing aid, amplified, and delivered to the hearing aid user along with the desired signal, such as the voice of a human speaker. In addition to digital cell phones, digital cordless phones, portable digital radios and other digital devices generate electromagnetic interference which, when processed by the hearing aid, is subsequently output to the user. Analog apparatus such as power transformers, fluorescent lighting, and power lines likewise produce electromagnetic field static that interferes with hearing assistive devices.
Electronic interference, whether generated by pulsating electric or magnetic fields, combines with the desired signals picked up by a microphone, telecoil, or circuitry to form a composite signal at the hearing assistive device. The composite signal is processed by the hearing assistive device and output to the user. Depending on the source and duration of the interference, the hearing assistive device performance may be noticeably and significantly reduced, to the point where the hearing impaired user is discouraged from either using the hearing assistive device, such as a hearing aid, or discouraged from using the item that generates the interference, such as a cellular telephone.
Due to the importance of accessible wireless communications for the hearing impaired, the Federal Communications Commission (FCC) has imposed a mandate for hearing aid compatibility (HAC) for wireless communications devices on both device manufacturers and service providers. Many of the solutions proposed for meeting these HAC requirements have focused primarily on handset-based solutions.